This invention relates to methods and apparatus for pressure infiltration casting. More particularly, this invention relates to improved methods and apparatus for high throughput pressure infiltration casting.
Various techniques for casting molten metals and metal-matrix composites have been developed. Gravity casting, permanent mold casting, die casting, investment mold casting and squeeze casting commonly are exploited. However, pressure infiltration casting offers advantages over these methods. Besides overcoming the non-wettability of molten metal with a reinforcement, i.e., a preform, and the ability to rapidly prototype components prior to large scale production, pressure infiltration casting can produce near-absolute net-shape cast parts with low to negligible porosity. As a result, pressure infiltration castings are used in automotive, truck, heavy construction equipment and outboard motor applications. Pressure infiltration castings also may be used in aerospace and sports applications.
Pressure infiltration casting generally is a process where a pressure differential is used to move molten infiltrant into a mold cavity to produce a conventional monolithic casting, i.e., an unreinforced casting, having the shape of the mold cavity. Pressure infiltration casting also includes moving a molten infiltrant into a mold cavity containing a preform. A preform typically is another metal or ceramic, usually of a particular shape and size such as a fiber. A reinforced casting, e.g., a metal-matrix composite, results from infiltration of a preform.
Pressure infiltration casting processes typically evacuate a mold cavity before addition of molten infiltrant to reduce or eliminate porosity of the finished product due to trapped air. Using the proper techniques, pressure infiltration casting can produce net shape reinforced composites or conventional castings with dimensional tolerances of xc2x10.0002 inches with a surface finish of 4 microinches (about 0.1 xcexcm), i.e., a surface with a mirror-like finish.
The overall pressure infiltration casting process generally involves the steps of (1) heating a mold vessel containing a mold; (2) heating an infiltrant to a molten state; (3) evacuating the heated mold vessel; (4) adding the molten infiltrant to the evacuated heated mold vessel if not initially present in the mold vessel; (5) applying pressure to the molten infiltrant to move it into a mold cavity; and (6) solidifying the molten infiltrant to form a finished cast product. Certain of the above steps may be conducted simultaneously and in the same vessel. For example, the mold vessel and the infiltrant often are combined and heated in the same chamber of an apparatus, as are the steps of pressurizng and cooling often conducted in the same chamber, usually different from the heating chamber.
Heating the mold vessel, mold and infiltrant usually requires the greatest amount of time in the overall casting process. Infiltration of the mold cavity with the molten infiltrant typically is the fastest step, while solidification of the molten infiltrant in the mold takes longer than infiltration but less time than heating the mold vessel and infiltrant. Accordingly, the throughput of finished products, i.e., the number of parts cast per unit time, may be increased by shortening the length of time for an individual step in the overall process or by strategically segregating steps so certain tasks may be performed simultaneously.
Early pressure casting publications and patents generally disclose processes that use a one chamber apparatus to perform the whole casting process, i.e., heating, evacuating, adding infiltrant, pressurizing and cooling. See, e.g., U.S. Pat. No. 3,547,180 to Cochran, and U.S. Pat. No. 3,913,657 to Banker et al.; and DE 3603 310 A1 to Zapfe. State-of-the-art publications and patents generally disclose processes that use multi-chamber apparatus where typically the steps of heating and evacuating are separated from the steps of pressurizing and cooling. See, e.g., U.S. Pat. No. 4,832,105 to Nagan et al., and U.S. Pat. No. 5,335,711 to Paine; and DE 3220 744 A1 to Reuter et al. and GB 2,195,277 A to Doriath et al. However, state-of-the-art processes typically heat and evacuate a mold vessel and infiltrant in the same chamber.
In the aforementioned processes, the one chamber or multi-chamber apparatus is in use during the full casting cycle thereby occupying the entire apparatus for every step of the process. Since the entire apparatus is in use even during the slowest steps of heating and cooling, expensive vacuum and pressure equipment and chambers are used for only a short period of time. Thus, state-of-the-art pressure infiltration casting processes, even using multi-chamber apparatus, have a limited throughput because of the heating, and to a smaller degree cooling, steps.
It had been discovered that the steps of heating and evacuating may be conducted in a vessel separate from pressuring and cooling, however, these methods typically require the use of a vent tube. See, U.S. Pat. Nos. 5,322,109 and 5,553,658 to Cornie, which are herein incorporated by reference in their entirety.
Additionally, state-of-the-art pressure infiltration casting solidification methods generally involve using heat sinks, a chill zone or chill plate. See, e.g., U.S. Pat. No. 3,770,047 to Kirkpatrick et al.; U.S. Pat. Nos. 5,111,870 and 5,111,871 to Cook; and U.S. Pat. No. 5,275,227 to Staub. A chill plate often is made of metal in the shape of a pedestal which is brought into contact with a heated mold vessel after pressure has driven the molten infiltrant into the mold cavities. The chill plate also may have active means for facilitating the heat transfer process such as fluid flowing through the interior of the chill plate or through coiled pipes. Since cooling tends to be the second longest step in the pressure infiltration casting process, state-of-the-art solidification techniques also limit the overall throughput of the pressure infiltration casting process.
Accordingly, there exists a need for improved methods for pressure infiltration casting which economically produce with increased throughput high quality cast parts. In addition, there exists a need for improved apparatus for conducting high throughput pressure infiltration casting.
It is an object of this invention to provide an economical method for high throughput pressure infiltration casting which uses a mold vessel as an evacuation chamber to produce superior quality finished cast parts. It is another object of this invention to provide a method for high throughput pressure infiltration casting where the molten infiltrant is directionally solidified at an increased rate by using an improved heat extraction technique. It is a further object of this invention to provide apparatus for practicing methods for high throughput pressure infiltration casting. Apparatus include a removable evacuation cap in conjunction with a fill tube and a mold vessel/evacuation cap assembly which uses the mold vessel as an evacuation chamber.
The invention provides a pressure infiltration casting process which operates at the limits of processing time. High throughput is achieved in part by heating and evacuating a mold vessel containing a mold separate from heating the infiltrant. Accordingly, a dedicated source of molten infiltrant can be maintained while mold vessels are heated and staged while waiting to be evacuated and charged with molten infiltrant.
Subsequent to charging molten infiltrant to an evacuated mold vessel, the heated mold vessel containing molten infiltrant is transferred to a dedicated pressure vessel which typically contains means for cooling the molten infiltrant. Certain methods of the invention provide an improved solidification technique which increases the rate of directional cooling by using a low melting temperature material. Thus, the pressure infiltration casting methods of the invention strategically segregate the time restrictive tasks of the overall process to separate steps which simultaneously can be conducted. In particular, heating the mold vessel and infiltrant independent of the other steps avoids occupying vacuum and pressurizing equipment during the whole casting cycle.
Methods of the invention for pressure infiltration casting generally involve providing a mold vessel which houses a mold having a mold cavity. The mold cavity may contain a preform which will produce a reinforced casting. The mold cavity, optionally containing a preform, is evacuated using a vacuum source. A charge of molten infiltrant not in vacuum communication with the mold vessel then is added into the mold vessel while maintaining a reduced pressure, i.e., a vacuum, in the mold cavity.
An infiltrant separately is heated to form a molten infiltrant usually in a infiltrant heating vessel such as a crucible, also not in vacuum communication with the mold vessel. Subsequent to transporting the molten infiltrant into the mold vessel, pressure is applied to the molten infiltrant to move it into the mold cavity and preform, if present. Finally, the molten infiltrant is cooled in the mold cavity to produce a solidified finished cast product that can be recovered from the mold.
In certain embodiments of the invention, the method may involve the additional steps of heating a mold vessel to produce a heated mold vessel and insulating the heated mold vessel to produce an insulated heated mold vessel. Following addition of a charge of molten infiltrant into the mold vessel, the insulated heated mold vessel typically is transferred to a pressure vessel. In the pressure vessel, pressure is applied to drive the molten infiltrant into the mold cavities. If a low porosity finished product is desired, pressure may be applied continuously to the molten infiltrant during the cooling step to produce a high density, near net-shape cast part.
In other embodiments of the invention, the molten infiltrant is directionally solidified which may involve a low melting temperature material to increase heat transfer away from the molten infiltrant. The low melting temperature material has a liquid heat transfer zone which creates a liquid/solid interface with a heat transfer surface. The heat transfer surface, which is in thermal communication with molten infiltrant within a mold cavity, is exposed to the liquid heat transfer zone to solidify the molten infiltrant. The liquid heat transfer zone may be present prior to thermal communication with the mold vessel and mold or may form upon contact of a heated mold vessel with the low melting temperature material. Preferred low melting temperature materials include, but are not limited to, metals, metal alloys, salts and organic materials. Preferred metals or metal alloys are aluminum, antimony, bismuth, cadmium, gallium, indium, lead, tin, zinc, solder, woods metal and mixtures thereof.
In other embodiments of the invention, a high melting temperature material in thermal communication with the low melting temperature material may be used during the cooling step to more economically and/or efficiently facilitate heat transfer. Alternatively, an active cooler, e.g., piping having a cooling fluid pumped therethrough, may be used independently or with a low melting temperature material and/or high melting temperature material to further reduce the amount of low and/or high melting temperature material required.
The ratio of the amount of low melting temperature material and/or high melting temperature material to the amount of molten infiltrant should be at least equal to the ratio of the latent heat of fusion of the low melting temperature material and high melting temperature material to the latent heat of solidification of the molten infiltrant. Preferably, the ratio of the amount of low melting temperature material and/or high melting temperature material to the amount of molten infiltrant is at least 90%, and more preferably at least 75-80%.
In other embodiments of the invention, the step of transporting a charge of molten infiltrant into a mold vessel involves opening a vacuum seal. The vacuum seal may be a valve or other means for sealing a vacuum in the mold vessel. The same or a second vacuum seal also may control the flow of molten infiltrant.
In another aspect of the invention, apparatus for high throughput pressure infiltration casting are provided. One embodiment of an apparatus of the invention is a removable evacuation cap that permits a mold vessel to be evacuated and filled with molten infiltrant. By methods of the invention, the need for expensive vacuum chambers is eliminated since the mold vessel in essence becomes the vacuum vessel. Moreover, since the mold vessels and evacuation caps can be reused, production costs are reduced further.
The evacuation cap has a housing which has an interior surface and an exterior surface. The interior surface forms a seal with a mold vessel to allow reduced pressure to be realized in the interior space of the mold vessel. The evacuation cap also has at least one port extending through the housing which permits fluid communication through the housing. The port permits at least a vacuum source to communicate through the housing of the evacuation cap.
In another embodiment of the invention, the port of the evacuation cap also permits molten infiltrant to be charged to the interior space of the mold vessel. The apparatus typically has a vacuum seal in communication with the port to independently isolate a vacuum source and molten infiltrant from the interior of the mold vessel. The vacuum seal may be a vacuum sealing material, a valve or similar flow control device. A quick release or disconnect connection may be situated in a port to permit easy and efficient connection to a vacuum source or molten infiltrant source.
In another embodiment of the invention, the evacuation cap has at least a second port so the mold vessel is evacuated using one port and molten infiltrant is charged into the mold vessel through an independent second port. The apparatus may have a first vacuum seal in communication with the first port and a second vacuum seal in communication with the second port. The vacuum seals independently isolate the vacuum source and the molten infiltrant from the interior of the mold vessel. As above, the vacuum seals may be a vacuum sealing material, a valve or similar flow control device.
In yet other embodiments of the invention, the evacuation cap has a vacuum gasket contacting an interior surface of the evacuation cap. When the evacuation cap is sealed against the mold vessel, the vacuum gasket assists achieving and maintaining a vacuum in the mold vessel interior. The evacuation cap also may have an insulator on an interior surface of the evacuation cap. The insulator usually is in communication with the interior of the mold vessel when the evacuation cap is in use. The insulator helps prevent overheating of the evacuation cap and its components, e.g., analytical devices and gauges such as thermometers and/or manometers, electronic devices, gaskets, seals and the like. The evacuation cap also may have a cooler to assist in cooling the evacuation cap and its components to increase the functional lifetime of the evacuation cap.
In other preferred embodiments of the invention, the apparatus includes a fill tube or xe2x80x9csnorkelxe2x80x9d which has a first end in communication with a port of the evacuation cap. The fill tube has a second end which has a vacuum seal such as a vacuum sealing material, valve or similar flow control device. In preferred embodiments, the vacuum sealing material at the second end of the fill tube is meltable. In practice, the second end of the fill tube communicates with a source of molten infiltrant so molten infiltrant is charged into the mold vessel, sealing a vacuum in the mold cavities.
Another embodiment of an apparatus of the invention has an evacuation cap which may be sealed against a mold vessel. The evacuation cap and mold vessel independently may have one or more ports therethrough (although note that only one port is required in either location to practice the invention). In preferred embodiments, more than one port is present. The interior space of the mold vessel contains a mold which has a mold cavity. An evacuation cap sealed against a mold vessel isolates with the interior of the mold vessel, i.e., interior space, from its surrounding environment and permits efficient evacuation of the mold cavity. In a preferred embodiment of the apparatus, the evacuation cap is removable to allow the mold vessel to be independently transferred to a pressure vessel so the evacuation cap can be used with the next mold vessel/molten infiltrant assembly of the casting cycle production process. However, another embodiment of the apparatus has an evacuation cap permanently mounted on the mold vessel.
In embodiments containing a mold vessel, evacuation cap and one or more ports, the port(s) are positioned above the mold cavity and permit communication of the interior space of the mold vessel with the exterior of the mold vessel. The port(s) communicate through the evacuation cap and/or through a mold vessel wall. For example, the mold vessel may have the only port present for a particular embodiment of the invention or may have two or more ports. In addition, each of the evacuation cap and the mold vessel may have one or more ports. However, in a preferred embodiment of the invention, one or more ports are positioned through the evacuation cap.
It should be understood that the apparatus including the mold vessel/evacuation cap assembly may include any number or all of the previously described embodiments associated with the evacuation cap.
Reference to the figures are intended to provide a better understanding of the methods and apparatus of the invention but are not intended to limit the scope of the invention to the specifically drawn embodiments. Like reference characters in the respective drawn figures indicate corresponding parts. In addition, it should be understood that the individual steps of the methods of the invention may be performed in any order and/or simultaneously as long as the invention remains operable.
The invention will be understood further from the following drawings, description and claims.